1. Field of the Invention
The present invention relates to hybrid aircraft capable of operating as a rotary wing aircraft and as a fixed wing aircraft.
2. Background of the Invention
A functional, preferably single rotor, hybrid aircraft capable of carrying a plurality of passengers with a substantial payload has not been successfully developed. The military, commercial and private benefits of a vehicle that can take off vertically, transition from low to high speed forward flight and back to low speed flight for vertical landing are well known.
Vibration, structural and aerodynamic barriers presently practically prevent the use of high speed rotary wings. The primary barrier to high speed helicopter flight is that the retreating rotor blade in high speed forward flight will stall because its effective airspeed approaches zero at a set rotational velocity. Conversely, the advancing rotor blade sees a higher airspeed. Because lift vanes as the square of airspeed, the lower airspeed on the retreating blade requires a larger pitch angle of attack than the advancing blade. As the retreating blade airspeed velocity vector sum approaches zero the pitch angle will approach an angle of attack where a blade stall will occur. One way to avoid these stall barriers is stopping the rotor blades in flight when a sufficient forward air speed has been obtained.
The challenge in stopping the rotor is similar to that of high speed flight. Sufficient forward speed of the rotor relative to the aircraft must be achieved and maintained to lift the aircraft as the rotor is stopped. The retreating blade will see a reduction in airspeed to zero and then the airflow will reverse before the rotor system stops. Because of this the control sequence for the retreating blade must also be reversed while the rotor system is rotating before it is stopped.
There will be a period of time when the retreating rotor blade cannot provide any control or lift to the aircraft because of its near zero relative airspeed. The range of vehicle airspeed in which the rotor blade cannot provide control or lift is bound by the retreating rotor blade stall speed, and its stall speed when the airspeed reverses as the net airspeed increases and the blade rotation slows. For control, the aerodynamic lift characteristics must be the same in both directions, therefore the blade must be effectively symmetrical in cross section at all times.
The aerodynamic requirements of high speed fixed rotor flight contrast with the aerodynamics of low speed rotary wing flight. During fixed rotor flight a given surface area is required at a given airspeed to avoid stalling. The slower the airspeed the larger the surface area required. It is advantageous to transition from fixed wing flight to rotational flight at low to moderate airspeeds for control and structural reasons. Therefore, a larger surface area is preferred. However, during rotational flight higher airspeed is typically seen over the rotor blades because of the added angular velocity. Thus a smaller surface area is sufficient to provide lift. Therefore, a larger surface area during rotational flight is detrimental to efficiency and control responsiveness.
Taylor in U.S. Pat. No. 3,253,805 describes an annular lifting surface with an airfoil shape. The annulus is attached to the aircraft and acts as a fixed wing and does not provide for rotation of the wing. Furthermore, the annulus is drawn with a substantially larger forward airfoil section than the rear section. This configuration is not suited for rotation due to the eccentricity of the shape and corresponding asymmetrical moment of inertia.
Binden in U.S. Pat. No. 4,711,415 describes rotor locking with an X-wing aircraft. The X-wing aircraft disclosed therein employs compressed air slots which modulate to control the rotor profile in rotational, transitional and locked rotor flight. This mechanism was investigated throughout the 1980's, but was not commercially introduced, possibly due the complexity and cost issues of the test program. A particularly challenging issue in this design is the forward swept nature of the aft rotor blades which act as a primary lifting surface in forward, fixed-wing flight. Instability with aerodynamic surfaces of this nature are well known and place demanding loads both on structure and on flight control mechanisms.